Damage to silicon making up semiconductor devices, such as photodiodes, is a well known phenomenon creating noise and other inaccuracies in image acquisition devices. Damage to the crystalline structure of a semiconductor can cause dark current and white pixel distortion in photodiode operation by altering the structure of the silicon in the photodiode. In photodiodes, silicon damage may cause the photodiode to improperly promote spurious electrons not associated with photoreactivity. Such spurious electrons cause a photodiode to register a light reading, even when no light has caused the promotion of an electron. Thus, semiconductor damage can cause inaccuracies in a photodiode's sensing of light by inaccurately reflecting the actual amount of light sensed by the photodiode, introducing noise into an image. Dark current is current generated without light, while a white pixel defect is damage to, or overloading of a photodiode by excess dark current, causing the photodiode to always read as if it had sensed a pure white light, when it had not.
Photodiodes such as complimentary metal oxide semiconductors (CMOS) diodes are commonly used for sensing images in cameras and other video or photo devices. Recently, photodiode devices have been improved by using backside illumination (BSI). Generally, photolithography processes deposit structures such as gate oxides, metal interconnects, and the like, on the top side of silicon wafer or other substrate. Early photodiodes gathered light from the top, the same side where the device structures were applied. Metal interconnects deposited on the top surface of the photodiode substrate can block portions of the photosensitive regions of the photodiode, degrading the picture quality and individual photodiode sensitivity. BSI is the collection of light from the backside of the photodiode substrate, with interfering structures such as metal interconnects, gate oxides or the like deposited onto the top side of the substrate, and then the substrate abraded or otherwise thinned to allow light to pass though the substrate and affect the photosensitive region of the photodiode. Ideally the substrate thickness is reduced so that light may enter the backside of the device and strike the photosensitive region of the photodiode, eliminating obstruction and interference during image capture from deposited structures and metal interconnects.
Frequently, the thinning of a photodiode to permit BSI operation is done through chemical mechanical polishing. However, on a sub-nanometer scale the surface of even a highly polished substrate may have irregularities, crystal deformities, or the like, resulting dangling electron bonds. The loosely bound dangling electronics can lead to instances of dark current and white pixel conditions. Thus, reductions in surface irregularities resulting in reduced dark current and white pixel anomalies and more accurate imaging devices.